Novel critical behavior of magnetization in URhSi:Similarities to uranium ferromagnetic superconductors UGe$_2$ and URhGe

Abstract

We study the critical behavior of dc magnetization in the uranium ferromagnet URhSi around the paramagnetic to ferromagnetic phase transition at T_C~ 10 K with a modified Arrott plot, a Kouvel-Fisher plot, the critical isotherm analysis and the scaling analysis. URhSi is isostructural to uranium ferromagnetic superconductors URhGe and UCoGe. The critical exponent beta for the temperature dependence of the spontaneous magnetization below TC, gamma for the magnetic susceptibility, and delta for the magnetic isotherm at T_C in URhSi have been determined as beta = 0.300 +- 0.002, gamma = 1.00 +- 0.02, and delta = 4.38 +- 0.04 by the scaling analysis and the critical isotherm analysis. These critical exponents fulfill the Widom scaling law delta = 1+ gamma/beta. Magnetization has strong uniaxial magnetic anisotropy in the ferromagnetic state of URhSi. However, the universality class of the ferromagnetic transition does not belong to the 3D Ising system with short-range exchange interactions between magnetic moments (beta = 0.325, gamma = 1.241, and delta = 4.82). The obtained exponents in URhSi are similar to those in the uranium ferromagnetic superconductors UGe_2 and URhGe, and uranium ferromagnets UIr and U(Co_0.98Os_0.02)Al. We have previously reported the unconventional critical behavior of magnetization in the uranium ferromagnetic superconductors [N. Tateiwa et al. Phys. Rev. B 89, 064420 (2014)]. The universality class of the ferromagnetic transition in URhSi may belong to the same one in the uranium ferromagnetic superconductors and the uranium ferromagnets. The unconventional critical behavior of the magnetization in the uranium compounds cannot be understood with previous theoretical interpretations of critical phenomena. The absence of the superconductivity in URhSi is discussed from several viewpoints.